A drift eliminator has the function of removing water droplets, entrained typically as a mist which is referred to as “drift”, from the flow of air moving through the drift eliminator. The drift eliminator is utilized in evaporative cooling devices, such as cooling towers, to reduce the water volume within the air flow being discharged from the cooling device so that the circulating water is not lost from the evaporative cooling device. By retaining the circulating water within the evaporative cooling equipment, the drift eliminator allows the evaporative cooling device to retain most of the circulating water, as well as the water treatment chemicals within the circulating water.
Drift eliminators are normally formed as a stack of shaped or formed members that cause the air flow moving through the drift eliminator to travel along a curved path. The curved path creates changes in direction for the air flow that result in the water droplets being removed from the air flow. The water droplets will impact the curved walls of the drift eliminator and flow by gravity to the lower end of the drift eliminator stack and be discharged into the circulating water of the evaporative cooling device. Some water droplets continue within the air flow and are discharged from the drift eliminator, and ultimately from the cooling device.
“Drift rate” relates to the amount of water droplets that are carried out of the tower with the air. Drift rate is quantitatively measurable and is commonly expressed as a percentage of the circulating water flow in a tower. A critical performance criterion for drift eliminators is the velocity of the air flow moving through the drift eliminator. If the air velocity exceeds the rate for which the drift eliminator is designed to operate, the drift rate increases causing the drift eliminator to fail by exceeding the specified drift rate, i.e. allowing excessive drift to be discharged from the evaporative cooling device. The maximum operable air velocity rate is a function of the geometry of the drift eliminator, the proximity of the eliminator to the tower's water distribution system, the circulating water flow inside the tower and other factors.
Generally, conventional drift eliminators can be formed in a parallel blade configuration or in a cellular configuration. Parallel blade drift eliminators are constructed from a number of parallel curved blades separated by discrete spacers, which may be separate items or integrally formed in the blade. Cellular drift eliminators are formed from a number of curved blades separated by corrugated spacers that form tube-like cells through which the moisture-laden air flow moves. The parallel curved surfaces created from the stacked blades and corrugated spacers define impingement surfaces to separate water droplets out of the air flow. The tubular design of the cellular drift eliminator configuration adds strength to the stacked assembly. Cellular drift eliminators typically have a higher drift removal efficiency than parallel blade eliminators, but at a slightly higher pressure drop and thus require more power for a fan to move air through the drift eliminator.
Cellular drift eliminators are normally flat on the top and bottom with the walls of the cellular tubes terminating in a generally common horizontally extending plane. The cellular configuration provides increased strength over the parallel blade configuration. As is identified in U.S. Pat. No. 6,315,804, issued to Randall Bradley on Nov. 13, 2001, the planar configuration at the lower ends of the cellular tubes subject the tubes to being breached across the opening of the cellular tube by a film of water from the water droplets falling along the walls of the cellular tubes. The surface tension of these water blockages is sufficient to require an increase in power to move the air flow through the drift eliminator. One solution to this problem is disclosed in aforementioned U.S. Pat. No. 6,315,804, which is to cut a notch into the side wall of the cellular tubes so that water droplets cannot film across a horizontally planar opening.
A drift eliminator formed from curved blades that are stacked and glued together is disclosed in U.S. Pat. No. 4,500,330, granted to Wilson Bradley, Jr. on Feb. 19, 1985. In this patent, the drift eliminator blades are formed from suitable polymeric material, such as polyvinylchloride (PVC), into which is formed impact members to assist in the removal of water droplets from the air flow through the drift eliminator. In U.S. Pat. No. 7,105,036, granted on Sep. 12, 2006, to Gregory Shepherd, a drift eliminator is formed from a plurality of corrugated blade members in a stacked configuration to define cellular tubes for the movement of air through the drift eliminator. The corrugated blades are stacked in a manner to place troughs together so that the troughs can be bonded together by glue or other appropriate adhesive to form the cellular passageways.
The approach taken in aforementioned U.S. Pat. No. 6,315,804 requires the formation of the polymeric corrugated blade member, which can be formed through a thermoforming process in which a flat sheet of PVC film is heated and vacuum formed into the corrugated structure disclosed therein. A subsequent manufacturing step is then required to cut the arch or notch into the side wall of the blade corrugations. In this manner, the notch in the side wall will prevent the formation of the film of water over the inlet portion of the cellular tube, while maintaining a planar surface for support of the drift eliminator.
It would be desirable to provide a blade configuration that will prevent the formation of a film of water across the inlet opening of the cellular tube passageways for the flow of moisture laden air through the drift eliminator without diminishing the strength of the stacked structure.